Though the process of the invention has wide application for making virtually an infinite variety of solid solutions, it was developed initially for the purpose of providing a reliable and economic source of finely divided, uniformly sized particles of true solid solutions for use in making monolithic capacitors.
Monolithic capacitors comprise a plurality of dielectric layers, at least two of which bear metallizations (electrodes) in desired patterns. Such capacitors are made by either the "green tape" process or by the the thick multilayer process. In the former, multilayer capacitors are made from a green (unfired) tape of particles of dielectric materials held together with an organic binder by cutting pieces of tape from a sheet of tape, metallizing some of the tape pieces, stacking and laminating the pieces of tape, cutting the laminate to form individual capacitors and firing the resultant individual capacitors to drive off the organic binder and vehicles and to form a sintered (coherent) body. In the latter method, multilayer capacitors are made by printing and drying alternating layers of thick film conductor materials and dielectric material in a desired configuration on a rigid ceramic substrate such as Al.sub.2 O.sub.3. The sequence of steps is repeated until the desired number of capacitor layers is fabricated. The dried stack of capacitor layers is then fired in the same manner as the green tape to effect densification of the dielectric material.
Metallizations useful in producing electrodes for capacitors usually consist of finely divided metal particles applied to the dielectric green tapes in the form of a dispersion of such particles in an inert liquid organic medium or vehicle. Selection of the composition of the metal particles is usually based on a compromise of cost and performance. Since base metals often are oxidized in air at elevated temperatures and/or in many cases react with the dielectric material during firing, noble metals are usually preferred because of their relative inertness during firing of the laminates to produce electrically continuous conductors. By far the most widely used monolithic capacitor electrode materials have been palladium and mixtures of finely divided palladium and silver powders which become alloyed during firing.
In the fabrication of multilayer capacitors, the required solidus temperature of the electrode metal will ordinarily be determined by the sintering temperature of the dielectric material on which it is applied. In turn, the sintering temperature is determined by the physical and chemical characteristics of the dielectric material. Thus, to prevent excessive diffusion of the conductive metal into the dielectric layer during sintering, it is preferred to employ a metal or solid solution of metals having a solidus temperature higher than the firing temperature and preferably at least 50.degree. C. higher.
Silver would most frequently be the noble metal electrode material of choice because it has both suitable conductive properties and the lowest cost of the noble metals. However, when used in capacitors, the electrode material is subjected to firing temperatures of 1100.degree. C. or even higher. These temperatures are necessary for most state-of-the-art dielectric materials to be sufficiently sintered to obtain suitable densification and dielectric properties. Because metallic silver melts at only about 961.degree. C., silver metal alone would melt completely at 1100.degree. C. or higher and be of such low viscosity that it would too readily diffuse into the dielectric material and thus substantially degrade the capacitor properties of the sintered dielectric material. For this reason, it is preferred to employ a solid solution of palladium and silver which has a suitably high solidus temperature that it cannot migrate significantly into the dielectric material at normal firing temperatures. This solid solution or alloy is best provided as an already formed Pd/Ag alloy for the reason that any unalloyed silver would be available to migrate into the dielectric layer.
As a result of the above considerations, for those X7R and NPO class capacitors which are fired at 1100.degree., a 30/70 Pd/Ag mixture is used. On the other hand, for a Z5U class capacitor which is fired at 1450.degree. C., a 70/30 Pd/Ag or 100 Pd mixture is used as the noble metal component. In general, the ratio of palladium to silver is dependent upon the maximum firing temperature which is used to densify the dielectric material to a well sintered body.
The particles of the metal component of the metallization should be sufficiently small that the paste can be used even in conventional screen printing operations and that the particles can be readily sintered. Furthermore, in the production of capacitors from green dielectric sheets, the presence of coarse particles in the inner electrode prints must be avoided lest they cause puncturing of the green dielectric sheets. Generally, the metallizations are such that at least 90% of the noble metal particles are no greater than 8 microns in diameter; that is, in general their largest dimension should be no greater than 8 microns. However, when the thickness of the green dielectric layer is less than 1 mil, the particles must be correspondingly smaller.
The problem of making such particulate material as true solid solutions is illustrated quite graphically by reference to U.S. Pat. No. 3,390,981 to Hoffman. Based on a U.S. patent application filed in 1963, the patent purports to cover a process for producing solid solution particles of two noble metals by treating a solution of the metals with a reducing agent capable of simultaneously reducing the metal constituents to the corresponding metals. The process was carried out with at most mild agitation (rapid stirring, Example 1) and slow addition of reductant. By virtue of a particularly described melting point determination, it was concluded that the particles therefrom were solid solutions. However, modern x-ray diffraction studies of products made in accordance with the teaching of the reference show separate Ag and Pd peaks, which indicate a mixture of the two metals and not a true alloy or solid solution. It has since been shown that the melting points obtained as described in the reference were the result of formation of a solid solution during the melting point test itself (See col. 5, lines 17-23 of the reference). Also of interest is U.S. Pat. No. 3,717,453 to Daiga which is directed to a method of forming an "homogeneous powder" of Ag, Au and another metal by coprecipitation of Ag and the other metal and then precipitating the Au from a slurry of coprecipitated Ag and other metal. The reference also discloses an alternative method which involves forming a solution of Ag and the other metal, adding Au powder and then precipitating the Ag and other metal from a slurry of the Au powder. While the Daiga process sometimes results in alloy formation, the process is neither intended to nor does it result in uniform alloy formation.